I use light mix and a coco mix with great success.
You need to treat it carefully.. as if you mix too much coco. You'll need to treat it so. Even though it says soil. It's mainly made of coco/perlite.
But, no, adding coco will only make it harder for your soil to hold water.
I've never had this issue. Yes, as the plant grows up. You've too treat it like coco, it more or less needs water everyday, "of course some feeds will be ferts". If you add some all mix. It should help the plant stay wet longer.